The demand for clean, secure, and renewable energy has stimulated great interest in fuel cells. Fuel cells are one distinct category of devices that are capable of converting chemical energy into electrical energy. Among the fuel cells that are currently under active development, the alkaline fuel cell, the polymeric-electrolyte-membrane fuel cell and the phosphoric-acid fuel cell all require essentially pure hydrogen as the fuel to be fed to the anode.
Solid Oxide Fuel Cells (SOFCs), on the other hand, are a type of fuel cells that use a solid, mostly ceramic and inorganic oxides, as the electrolyte of a cell. The solids typically are not conductive until they reach high temperature, but the high temperatures also allow reforming of low molecular weight hydrocarbons, therefore the fuel processing reaction can be carried out within the cell stacks without additional fuel processors. SOFCs thus offer great promise for the efficient and cost-effective utilization of a wide variety of fuels such as ethanol and methane, coal gas and gasified biomass.
The major hurdle to fuel flexibility is the vulnerability of the state-of-the-art Ni-YSZ (yttria-stabilized-zirconia) anode materials to coking and sulfur poisoning. In addition, the high operating temperatures of SOFCs, stemming from the low ionic conductivity of the electrolyte materials and the poor performance of the cathode materials at lower temperatures, increase costs and reduce the system operation life.
Thus, in order to make SOFCs fully fuel-flexible and cost-effective power systems, the issues of anode tolerance to coking and sulfur poisoning, slow ionic conduction in the electrolyte and sluggish kinetics at the cathode need to be addressed. In a broader scientific context, the chemical and electrochemical mechanisms that lead to both of these issues and the phenomena that could prevent them should be investigated in order to best optimize the materials and microstructure of SOFCs for excellent performance and stability.
Oxygen ion conductors have been the conventional conductors for electrolyte use in SOFC (see e.g. the reactions shown in Table 1). The prevailing material for an oxygen ion type solid electrolyte is yttria-stabilized zirconia (YSZ). Consequently, the high operating temperature of SOFCs is necessary because the ion conductivity is only satisfactory when the operating temperature is, for example, higher than 750° C.
However, today both proton and mixed ion conductors are also available for SOFC use. Proton-conducting electrolytes have the advantages of high proton conductivity and low activation energy at intermediate temperatures, which may widen the selection of materials to be used in SOFC. Additional advantages of proton-conducting electrolytes include water being generated in the cathode side of the SOFC, thus avoiding fuel dilution at the anode side. The reaction chemistry and examples of oxygen-ion conductors and proton conductors are shown in Table 1:
TABLE 1Oxygen ion and proton conductorsType of conductorOxygen ionProtonAnodeH2 + O2− → H2O + 2e−/H2 → 2H+ + 2e−CO + O2− → CO2 + 2e−CathodeO2 + 4e− → 2O2−2H+ + 2e− + ½ O2 → H2OOverall2H2 + O2 → 2H2O/2H2 + O2 → 2H2O2CO + O2 → 2CO2AdvantagesH2O, CO2 and high temperatures atNo fuel dilutionanode (fuel side) facilitates reforming ofIntermediate operating temperaturehydrocarbon fuels to H2 and CODisadvantagesHigh operating temperature degradesReforming at anode (fuel side) lostsystem components and adds to costH2O formed at anode dilutes fuelExamplesYttria-stabilized zirconia (YSZ)Y-doped BaZrO3 (BYZ)Samarium doped ceria (SDC)Calcium-doped lanthanum niobateGadolinium doped ceria (GDC)(LCaNb)Scandia stabilized zirconia (ScSZ)Y-doped BaCeO3 (BCY)Strontium and magnesium dopedBarium-zirconium-cerium-yttriumlanthanum gallate (LSGM)(BZCY)Yttrium- and ytterbium-doped barium-zirconate-cerate (BZCYYb)Scandia doped BZCY (BZCYSc)
The third option is to tailor the proton and oxygen ion transference number of the mixed ion conductor, allowing CO2 to form on the fuel side while allowing most of the H2O to form on the air side. The class of mixed proton and oxygen ion conductors holds great potential for a new generation of low temperature SOFCs. However, to date the ideal mixed ionic conductor has not been found.
The above-mentioned electrolytes generally have a perovskite structure with chemical formula ABX3, wherein the A and B atoms are cations with different sizes and X is an anion bonding to each cation. Usually the A atom is larger than the B atom, and the relative ion size is crucial to the stability of the resulting structure. To alter the physical and chemical properties of a perovskite substance, doping at either A or B site of the structure has been attempted.
Recent developments in solid electrolytes, especially in the area of increasing the ion conductivity at lower temperature, include reducing the thickness of the solid electrolyte so that the distance between the cathode and anode is shorter for the oxygen ions to travel. However, the thinner materials are more likely to break.
Other improvement includes changes of composition or doping with additional materials to increase the ion conductivity at lower temperatures.
For example, doped ceria is one of the most promising electrolyte materials that has the potential of sufficient ion conductivity at temperatures lower than 650° C. However, other issues of this material need to be addressed before it can be commercially employed, such as electric conduction and poor mechanical integrity.
Based on the fact that doped barium cerate exhibits a high ionic conductivity but poor chemical stability, while doped barium zirconate based materials have superior chemical and thermal stability but low conductivity, it has been proposed to replace a fraction of Ce in BaCeO3 with Zr. This type of solid solution is expected to exhibit high proton conductivity and excellent chemical and mechanical stability, as well as high ionic transference number over a wide range of conditions.
Yttrium- and ytterbium-doped barium-zirconate-cerate or “BZCYYb” is a mixed protonic and oxygen ionic conducting electrolyte that has demonstrated good conductivity. However, under most conditions, the proton conductivity is far greater than the oxygen ion conductivity. Furthermore, the material tolerates hydrogen sulfide in concentrations as high as 50 parts-per-million, does not accumulate carbon and can operate efficiently at temperatures as low as 500° C.
US20100112408 discloses the preparation of BaZr0.1Ce0.7Y0.2-xYbxO3-δ, by mixing all the ingredients followed by calcination. This preparation method, however, is not optimized to give the best performance of BZCYYb, and thus, there is considerable room for improvement.
Thus, what is needed in the art are better materials for use in SOFCs, which have both excellent ion conductivity at lower operating temperatures, but still maintain chemical and mechanical stability under the conditions of use.